In one aspect, the present invention relates to a method and instrumentation that measures, in real time, the density of a gas flowing in a pipeline. In this aspect, the invention can also measure a base condition density of the same gas where the base condition density corresponds to a density (i.e. mass/volume) of the gas determined as if the gas were at some defined base temperature and pressure condition. In a broader aspect, the present invention determines a ratio ##EQU1## of the measured flowing condition density compared to the base condition density. This ratio can be referred to as a volume correction ratio and can be used to translate a measured pipeline gas volumetric flowrate to a corresponding base condition volumetric flow rate.
Heretofore, accurate measurement of gas flowrates has often been difficult because accurate determination of gas density is important when measuring gas flowrates and gas supercompressibility effects gas density in ways that are not detectable by volumetric flowmeters. For instance, with differential pressure flowmeters (e.g., an orifice plate meter), the density term appears directly in the volume flow equation ##EQU2## Also, with other flowmeters (such as turbine flowmeters), as well as with differential pressure flowmeters, the translation of volumetric flowrate to base conditions (i.e. volume correction ratio) is directly related to the density of the flowing gas.
Yet, at elevated gas pressure, computing gas density from the ideal gas law is inadequate and can be substantially wrong because the ideal gas law does not properly account for supercompressibility. If gas properties are well known, the difficulty can be partially alleviated because gas supercompressibility can be directly computed and accommodated in the calculations. However, if the gas is a variable mixture of unlike molecules, gas properties necessary to compute supercompressibility are unknown and difficult to measure.
Gas densities, volume correction ratios, and volumetric flowrates at base conditions are normally calculated using flow computers from contemporaneous measurements of several gas parameters. Generally, for determining volumetric flowrates at base conditions, the pipeline gas volumetric flowrate at pipeline conditions is measured, the gas temperature and pressure at pipeline conditions are measured, and the composition of the gas is measured. Gas composition is normally measured by a chromatograph. From this measured data, gas supercompressibility at both pipeline and base conditions is calculated and from that the density of the gas at pipeline and base conditions is calculated. When operating pressure is elevated (and supercompressibility effects cannot be ignored) gas supercompressibility is usually estimated from either virial equations of state or from correlations such as NX-19. Using virial coefficients is severely limited because the virial coefficients are functions of temperature, pressure, and composition, are largely unknown, and have significant real time uncertainties. Also, using correlations such as NX-19 is often inaccurate because the correlations can be inaccurate for many compositions.
An alternate method for determining volumetric flowrates at base conditions involves the Gerg Equations. The Gerg Equations estimate supercompressibility and density from knowledge of the heating value, the density of the gas at base conditions and the percentage of carbon dioxide and nitrogen in the exhaust of burned gas. Such measurements can be made using PMI's GB 3000 product (Precision Measurements, Inc., Tulsa, Okla.). The Gerg Equations allow more rapid computation of supercompressibility than the above described composition methods and are, therefore, preferred in applications at normal natural gas pipeline pressures. However, as with composition methods, extensive use of a flow computer is required to solve the Gerg Equations.
Moreover, each of the measurements recited above in describing both the composition method or the Gerg Equations method introduce the potential for measurement error. The aggregation of such errors can substantially influence accuracy. Because of this, it is common practice to frequently calibrate and maintain each individual measurement device.
In co-pending patent applications, Ser. No. 07/793,753, filed on Nov. 18, 1991, and Ser. No. 07/787,188, filed on Nov. 4, 1991, inventions are disclosed that among other things can determine the volume correction ratio of a pipeline gas from energy type measurements. These inventions involve the measurement of energy flowrate and energy content of a sample of gas tapped from the pipeline. A base condition volumetric flowrate of the sample gas can then be determined by the ratio of the energy flowrate of the sample gas to the energy content of the sample gas. These inventions also measure the ratio of the mass flowrate of the pipeline gas in the pipeline compared to the mass flowrate of the sample gas tapped from the pipeline. The volume correction ratio (or the base condition volumetric flowrate of the pipeline gas) can be calculated easily from the mass flowrate ratio and the base condition volumetric flowrate of the sample gas.
While it is desirable to measure energy flowrate and energy content in many applications, in applications where energy flowrate or energy content are not required (but where base condition volumetric flowrates are desirable), the energy measurement equipment provides excessive expense. Further, the inventions in these two co-pending patent applications are directed to monitoring combustible pipeline gas and are thus inappropriate for monitoring flows of non-combustible gas.